The present invention relates to a multiple-color flexographic rotary printing machine.
In a flexographic printing process, colored ink is applied to a printing medium by means of three printing rollers or cylinders which rotate synchronously and in contact, with parallel rotation axes.
The three printing rollers comprise an impression roller, designed to support the material to be printed, a printing plate cylinder, which supports the pattern to be printed (printing plate), and an anilox or inking roller which applies ink to the printing plate. Printing occurs when the ink is deposited by the printing plate onto the medium, in an amount and in a manner which depend on the characteristics of the printing plate and of the anilox roller.
The color station is constituted by all the systems that move the three rollers, the ink and the material for printing a single color.
In current printing machines, the rotary motion of the three printing rollers is obtained by way of a mechanical transmission of the motion from a single electric motor; the electric motor transmits the motion by means of a reduction stage to the impression roller, which in turn transmits the motion to the printing plate cylinder by way of two gears which are axially aligned with the respective rollers and mesh together. A similar transmission system is adopted between the printing plate cylinder and the anilox roller.
In so-called xe2x80x9ccentral-drumxe2x80x9d printing machines, the impression roller is a single roller (central drum) for all the color stations and so is the ring gear that transmits the movement to the gears in axial alignment with the respective printing plate cylinders; the toothed ring has a peripheral diameter being equal to the diameter of the drum.
In order to eliminate the problems of gear-based transmission, systems have recently been studied and produced in which each printing roller is driven by its own electric motor, thus eliminating the mechanical connection of the gears between the printing rollers.
All the currently provided solutions are characterized by one or more transmission couplings and/or a motion reduction stage between the motor and the respective printing roller (the motion reduction stage designed to bring the rotation rate of the motor down to the rotation rate that corresponds to the speeds that the rollers must have for a correct printing process).
The reduction stage is generally constituted by a reduction unit or by gears being combined with a belt drive.
These technical solutions, which have transmission couplings and/or a motion reduction stage, have several disadvantages, the main ones being:
the play of the gears of the reduction stage (by means of reduction units or a belt) sets a limit to the precision of the printing registration of the colors;
the limited mechanical rigidity of a transmission system with transmission couplings and/or a reduction stage can cause vibration at low frequencies and can therefore facilitate the onset of resonance;
the manufacturing systems are mechanically complicated, wear easily and are also particularly expensive;
the transmission, in particular, is complicated and bulky, requires long assembly times and entails the use of precision mechanical components, which are expensive and delicate and require frequent maintenance.
The aim of the present invention is to eliminate or substantially reduce the above noted drawbacks, by providing a rotary printing machine with a central drum in which motion is provided to the central drum and to the printing plate cylinders without transmission couplings and/or motion reduction stages.
An object of the present invention is to provide a rotary printing machine which ensures a substantial increase in the torsional and flexural mechanical rigidity of the rotation shaft.
Another object of the present invention is to provide a rotary printing machine which raises the resonance frequency of the printing system.
Another object of the present invention is to provide a rotary printing machine which allows greater precision in printing registration, a structural simplification and a reduction in the mechanical parts that are subject to wear, so as to increase its reliability and at the same time reduce operating and maintenance costs, making them significantly more favorable than those of a conventional printing machine.
This aim and these and other objects which will become better apparent hereinafter are achieved by a rotary printing machine comprising two supporting shoulders, a central drum or cylinder being rotatably mounted on said supporting shoulders, at least one printing unit being arranged around said drum and comprising a printing plate cylinder and an anilox roller, which are rotatably mounted on a respective pair of supporting elements and whose rotation axis is parallel to the axis of said drum, characterized in that at least said central drum is actuated by a source of motion which directly engages an axial shank of said drum.
Conveniently, the source of motion for the drum has a stator being rigidly fixed to one of said supporting shoulders and a rotor being rigidly fixed at the end of an axial shank of the drum.
Advantageously, the rotary printing machine comprises a source of motion for the or each printing plate cylinder and for the or each anilox roller.
Conveniently, said source of motion for the or each printing plate cylinder comprises an electric motor in which the stator is rigidly fixed to one of said supporting elements and is monolithic therewith, and a rotor which is rigidly fixed to a motor shaft being coaxial to the respective printing plate cylinder and being slideable transversely to said stator and rigidly with the respective printing plate cylinder.